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   难度：Easy
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   <h1 class="question_title">
    404. Sum of Left Leaves
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    Find the sum of all left leaves in a given binary tree.
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    <b>
     Example:
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   <pre>
    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values <b>9</b> and <b>15</b> respectively. Return <b>24</b>.
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    404. 左叶子之和
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   <p>
    计算给定二叉树的所有左叶子之和。
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   <p>
    <strong>
     示例：
    </strong>
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   <pre>
    3
   / \
  9  20
    /  \
   15   7

在这个二叉树中，有两个左叶子，分别是 9 和 15，所以返回 24</pre>
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